Internally polygonal coil form

ABSTRACT

A low cost tubular coil form having regularly spaced flats on the inside surface thereof for improved tuning core retention.

United States Patent Renskers [4 1 June 20, 1972 [54] INTERNALLY POLYGONAL COIL FORM [72] inventor: John 0. Renskers, Crystal Lake, 111.

[73] Assignee: Coilcralt, Inc., Cary, lll.

[22] Filed:

[21] Appl.No.: 191,993

Oct. 26, I971 52 U.s.c|. ..336/l36 51 |m.c|. ..H0lf2l/06 [58] Fieldoi'Search ..336/130,l36,198

[56] References Cited UNITED STATES PATENTS 8/1961 Osborn et al. ..336/l36 9/1967 6/1970 6/l940 Curtis et al. ..336/l36 Kamenarouic ..336/ l 36 X Primary Examiner-Thomas J. Kozma Attorney-Gradolph & Rogers [57] ABSTRACT A low cost tubular coil form having regularly spaced flats on the inside surface thereof for improved tuning core retention.

4 Claims, 3 Drawing Figures 1 INTERNALLY POLYGONAL COIL FORM BACKGROUND OF INVENTION 1. Field of the Invention I This invention relates to molded tubular coil forms having improved means for tuning core retention.

2. Description of the Prior Art When nylon was the preferred material for molded tubular coil forms, the practise was to mold ribs on the inside surface of the form into which threaded tuning cores would be selftapping. Polypropylene has displaced nylon as the generally preferred material for these fomis because of its lower moisture absorption and thus more stable inductive values, but polypropylene lacks the abrasion resistance and the memory of nylon.

.Tuning cores in coil forms require a limited range of to:- sional resistance to movement; too little, and they willvibrate out of adjustment; too. much, and excessive load is placed on the connected points of the mounted coil in the circuit. For instance, a one-quarter inch core in its appropriate form should have a torsional resistance to movement of from 1 to 6 inchounces; a three-sixteenths core from one-half to 4 inchounces; a No. 6 core, from one-quarter to 2 inch-ounces.

When polypropylene forms, molded like nylon fonns with ribs on the inside, have threaded tuning cores turned into them, the polypropylene ribs wear, shred, and take permanent sets to the degree that a manufacturer cannot assume that the torsional friction is sufficient to meet the minimum specification set forth above. The very first insertion of the core produces unpredictable results. Furthermore, every subsequent adjustment further weakens the engagement such that an initially satisfactory coil fonn may soon fail.

SUMMARY OF INVENTION This invention contemplates the substitution of regularly spaced flats on the interior of the coil fonn for the previously employed ribs. The flats are fonned to generate an inscribed circle slightly smaller in diameter than the major diameter of the threaded core so that the core will be self-tapping therewithin. It hasbeen found that the lateral support of that part of the flats actually engaged by the threads of the core effectively eliminates the shredding, and the embossed threads in the flats provide asufficient' area of contact such that the cores retain the necessary frictional engagement over a virtually unlimited number of adjustments. Furthermore, the manufacturer can assume with certainty that the tunable coils will predictably meet specifications. v

Another highly desirable benefit'arises from this substitution. Co'il forms of this character are formed internally by meansof core pins which are necessarily long and slim and,

therefore, subject to breakage. As is well known, plastic molding dies of the simplest character are expensive; and their amortization is a major factor in the expense of a molded article. Because of their fragility, core pins have a short life. If their expense could be reduced, much cost could be taken out of the molded forms.

The ribs are formed by grinding grooves longitudinally in the surface of the pin. The depth of the groove is critical to stay within the torque limits. The ribs are commonly from 0.010 to 0.025 inches wide and deep. Such dimensions demand precise milling or grinding, and the error implicit in forming such grooves in such small stock affords a strong likelihood that the torque of adjustment will be outside specifications. Testing is essential, and reworking of the pins is commonly necessary. Variations in tuning core length, diameter, and material likewise introduce torque variations requirs ing adjustment or corrective grinding of the core pins. However, siting the pins on the same center, finding the grooves and working to the requisite depth in a milling or grinding type operation demands extraordinary care and precision and is necessarily expensive. On the other hand, the present invention permits a simple surface grinding to correct for adjustment torque error.

A further advantage is the push through" resistance. In adjusting or servicing television sets, for instance, there will be heavy-handed individuals who hear too hard on tuning cores and push the core axially through the form, stripping embossed threads, and losing in the process, adjustment torque, or at least, losing the area of fine-tune. The forms with interior flats have on the order of twice the push through resistance as the ribbed forms. Quarter-inch forms, otherwise identical, have been compared. The ribbed forms have 2 to 3 pounds resistance; the forms with flats have a resistance in excess of 5 pounds.

In short, the advantages of the flats are so pronounced, both in effectiveness and in cost, that the structure is advantageously employed with nylon and other materials as well as with polypropylene forms.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENT In the drawings is illustrated a tubular coil form 10 of the general type shown in US. Pat. No. 3,169,234 issued Feb. 9, 1964, to John O. Renskers, embodying the present invention. It consists of a tubular post 12 having a basic uniform diameter, inside and out, over its length. It carries an integral skirt 14 about its lower end perforated as at 16 for the reception of coil leads. A coil winding 18 is mounted on the post with the leads 20 thereof extended through appropriate holes 16 in the skirt 14. A threaded tuning core 22 of powdered iron, for ex ample, is screwed into the axial bore 24 of the post.

Under past practice there would be longitudinal ribs spaced about the inside periphery of the post proportioned to intercept the threads of the core 22, and these would be the lines of engagement of the core; in other words, the core would clear the walls of the tube without the presence of the ribs.

, In the present invention, flats 26 are substituted for the ribs. Four equal, equally spaced flats arranged in a square configuration appear to be the most workable arrangement.

A core pin with threeflats presents a somewhat more difficult grinding problem, particularly in the corrective grinding. More than four are not only unnecessary and more difficult to grind; the multiplication of bearing surfaces on the tuning core increases adjustment torque and imposes a greater precision on corrective grinding. To this extent, four flats are preferred, but a greater or less number is distinctly within the purview of this invention.

As a specific example a quarter inch powdered iron core three-eighths inch long, will have an adjustment torque of about 4 inch-ounces with the flats formed to result in a penetration of the slug threads into the flats of about 0.005 inches in a polypropylene form. It will have approximately the same adjustment torque with the flats formed to result in a penetration of about 0.004 inches in a 6/10 nylon fonn.

In the claims hereoflthe term flat" is to mean that surface defined by a plane parallel to the axis of a cylinder intersecting the cylinder along two spaced lines.

I claim:

I. A tunable coil comprising a tubular coil fonn of yieldable material, the central aperture thereof having at least three equal, equally spaced flats therein, a coil mounted on the exterior of said form, and a threaded tuning core adjustably contained in said aperture in variably coupled relationship with said coil, the major diameter of said core being slightly greater 2. The combination of claim 1 wherein the cross section of said central aperture is a regular polygon of alternate straight and convex segments.

3. The combination of claim 1 wherein there are four of said flats. 5

4. The combination of claim 2 wherein there are four of said straight segments. 

1. A tunable coil comprising a tubular coil form of yieldable material, the central aperture thereof having at least three equal, equally spaced flats therein, a coil mounted on the exterior of said form, and a threaded tuning core adjustably contained in said aperture in variably coupled relationship with said coil, the major diameter of said core being slightly greater than the inscribed circle defined by said flats to be selftapping therewithin, said flats consisting of planar surfaces extending longitudinally of said form at least over the range of adjustment of said core.
 2. The combination of claim 1 wherein the cross section of said central aperture is a regular polygon of alternate straight and convex segments.
 3. The combination of claim 1 wherein there are four of said flats.
 4. The combination of claim 2 wherein there are four of said straight segments. 